1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a method for synthesizing metal cyanometallates (MCMs) for use in battery electrodes.
2. Description of the Related Art
Large-capacity, cost-effective energy storage is the transformational technology needed to enable the large scale integration of renewable energy (e.g. wind, solar). The rechargeable battery offers efficient electrical energy storage (EES). The lithium (Li)-ion battery has been the leading option based on its performance, but it is too expensive for large-scale EES. Therefore, it is necessary to develop superior low cost, high-performance electrode materials to reduce the cost of lithium-ion batteries or to develop a new system of rechargeable metal-ion batteries to replace lithium-ion batteries.
Sodium and potassium-ion batteries have attracted a great deal of attention recently because the reserves of sodium/potassium are more plentiful than lithium in the crust of earth. This abundance makes it possible to develop low cost batteries for EES. However, it is impractical to copy the structures of lithium-ion (Li+)-host compounds for use as sodium-ion (Na+) or potassium-ion (K+)-host compounds. Sodium and potassium ions are much larger than lithium ions and severely distort the structures of the Li+-host compounds. Thus, it is very important that new Na+/K+-host materials be developed, with a large interstitial space into which sodium/potassium-ions can easily and reversibly move. Na+/K+-ions have been inserted into metal cyanide compounds. Transition metal hexacyanoferrates (TMHCFs) with large interstitial space have been investigated as cathode materials for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current of 17 C (1 C=150 milliamp hours per gram (mAh/g) [6-8]. In spite of this, the materials demonstrated low capacities and energy densities because: (1) only one sodium-ion can be inserted/extracted reversibly into/from Cu-HCF or Ni-HCF per formula unit, and (2) these TM-HCFs electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction.
In order to compensate for these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. When assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V delivered capacities of ˜110 mAh/g. Very recently, FeFe(CN)6.4H2O and Na0.61Fe[Fe(CN)6]0.94 were reported to exhibit high energy density and power density and good stability during cycling [11, 12].
FIG. 1 is a diagram representing a metal cyanometallate (MCM) open framework (prior art). All the compounds mentioned above can be categorized as MCMs with the general formula AXM1MM2N(CN)Z. The open framework structure of the transition metal MCM facilitates both rapid and reversible intercalation processes for alkali and alkaline ions (AX). Some typical “A” metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), etc. Typical M1, M2 materials include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, and Mg.
The MCM capacity is determined by the available A-sites in the compounds into which the alkali or alkaline ions can be inserted reversibly in the range of working voltages. From the electric neutrality point of view, the valences of M1 and M2 mainly contribute to the amount of the available A-sites. For example, 2 sodium-ions can be inserted/extracted into/from Na2MnFe(CN)6 between 2-4 V vs. Na° because the valences of Mn- and Fe-ions can change between +2 and +3, and its theoretical capacity is 171 mAh/g. Noteworthy is the fact that a greater number of metal-ions, “A”, result in MCMs able to deliver a higher capacity for use in a metal-ion battery with a non-metal counter electrode. In addition, in order to neutralize charges, the transition metals are kept at low valances. In typical MCM materials (AXM1NM2M(CN)Z.dH2O) the value of X is less than or equal to 1.
It would be advantageous if a synthesis process existed to increase the number of “A” metal-ions in MCM materials.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, journal of Power Sources, 79 (1999) 215-219.    [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48 (2012) 6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52 (2013) 1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2 (2011) 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letter, 11(2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159 (2012) A98-A103.    [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.    [10] S.-H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2 (2013) A39-A41.    [11] X. Wu, W. Den, J. Qian, Y. Cao, X. Ai, H. Yang, Single-crystal FeFe(CN)6 nanoparticles: a high capacity and high rate cathode for Na-ion batteries, J. Mater. Chem. A., 1(2013)10130-10134.    [12] Y.-G. Guo, Y. You, X.-L. Wu, Y.-X. Yin, High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries, Energy & Environmental Science, (2014) DOI: 10.1039/C3EE44004D